1. Field of the Invention
The present invention relates to a method and apparatus for decreasing the effects of metals deposited on catalysts during a process of conversion of hydrocarbons to lower boiling fractions.
2. Discussion of the Prior Art
Although it has long been considered desirable to process residuals in FCC units, one long-standing problem which must be overcome before catalytic cracking of residuals can be performed economically relates to the harmful effects of vanadium or other metals deposited from oil on the cracking catalysts during the conversion reaction. It is known that the harmful effects of such deposition onto FCC catalysts can be reduced by adding certain materials to the catalyst in the reactor, either by incorporating these materials into the catalyst composition itself or by adding them as separate particles. These added particles serve to trap or passivate the metals. U.S. Pat. No. 4,451,355 to Mitchell et al and European Pat. No. 112,601 to de Jong, the disclosures of which are incorporated by reference herein, illustrate such known processes.
Examples of materials that exhibit the desired trapping effect on vanadium are sepiolite and perovskite, as disclosed in the above-cited patents. These passivating or trapping materials prevent the vanadium or other metals from migrating within and among the catalyst particles to zeolite sites which the vanadium or other metals would otherwise destroy. These passivating materials, which are added to the catalyst, are sent to the riser, so that they are present in the reactor during the conversion reaction. However, these passivating materials have poorer cracking activity, as disclosed by European Pat. No. 112,601, and lower selectivity to liquid products, as is characteristic of non-zeolite substances, than the catalyst alone. These disadvantages partially offset some of the improvement in catalyst performance resulting from the reduced effects of vanadium or other metals. Due to these limitations, prior art techniques for reducing the effects of vanadium on FCC catalysts have not been particularly efficient or economical.